I turned the heating off at 8 pm, but wondered whether I should leave it on low overnight?

Personally I would just to keep the pipes from freezing.

I leave my open fire burning all night for the same reason

Leave it on, but turn down the thermostat to like 5 deg overnight or you'll wake up with "Central Heating Throat and Head Syndrome"

I hate heating durning the night, but them bloody pipes and it's -15 outside.

It is a well known fact that you will use more fuel heating a cold room / property from scratch, it is far better to leave the heating on a constant low temperature.

Just remember if something such as heat runs continuously it can maintain a temp, if you shut it off and start it up again tomorrow it will have to work harder to catch up to the desired temp before it can maintain it. So really it is 6 to one and half a dozen to another. I prefer to leave mine on a lower setting at night so that it maintains a semi warmer temp and doesn't have to play catch up later. Plus if everyone did this it would help with preventing power outages or brown outs during the cold winter months.

leave the heat on.